A film forming process is one of semiconductor manufacturing processes. The film forming process typically involves activating a processing gas in a vacuum atmosphere by, for example, converting it into plasma or thermally decomposing it; and depositing active species or reaction products on a substrate surface. A film forming apparatus for performing such a film forming process includes a mounting table disposed in a processing chamber which is configured as a vacuum chamber. As the mounting table, there can be used a ceramic plate which incorporates therein a temperature control unit for performing a temperature control of a substrate, the ceramic plate also serving as an electrostatic chuck by having a chuck electrode in a surface portion thereof.
FIG. 6A illustrates an example of such a mounting table 9. The mounting table 9 of FIG. 6A is formed of a ceramic plate disposed on a support 91 made of, e.g., aluminum. Installed inside the support 91 to penetrate it are, e.g., three elevating pins 92 for performing a transfer of a substrate 10 from/to a transfer mechanism (not shown). Further, a foil-shaped electrode 93 for attracting and holding the target substrate is embedded in the mounting table 9 formed of the ceramic plate.
Meanwhile, the film forming apparatus performs a so-called pre-coating process before the substrate 10 is mounted on the mounting table 9. The pre-coating process is a process for forming a film 100, which is identical with a film 100 to be formed on the surface of the substrate 10, on the surface of the mounting table 9 and the inner wall of a processing chamber (not shown). The reason why this pre-coating process is performed is as follows. If the inner wall of the processing chamber or the surface of the mounting table 9 is exposed, a rate of a thin film being attached to the inner wall and the like is high at an initial stage immediately after starting the film forming process. The attachment rate (a film forming rate), however, becomes stabilized after the attachment of the thin film to the inner wall and the like is made to some extent. Due to this discrepancy in processing environment, there is likelihood that several substrates 10 processed at the initial stage of the film forming process would not satisfy expected specifications. The pre-coating process is effective to avoid the possible problems as described above (see, for example, Japanese Patent Laid-open Application No. 2004-096060).
Further, during the film forming process, it might happen that components of the pre-coated films attached to the inner wall and so forth are dispersed to the surface of the substrate 10. Thus, it is required that the components of the pre-coated films are identical or similar to those of films to be processed (so that no affection is made on the films on the surface of the substrate 10 even if the pre-coated films attached to the inner wall are dispersed and mixed into the films on the surface of the substrate 10).
There is still another reason why the pre-coating process is performed on the surface of the mounting table 9. Aluminum nitride is widely used for the ceramic plate forming the mounting table 9. When the substrate 10 is lowered and placed on the mounting table 9 after being received by the elevating pins 92, the substrate 10 and the ceramic plate would be slightly scratched against each other due to slight horizontal misalignment between the substrate 10 and the mounting table 9. In such a case, the aluminum nitride is attached to the rear surface of the substrate 10. According to an analysis result of the present inventors, the quantity of aluminum attached to the rear surface of the substrate 10 was about 5×1010 atoms/cm2. Though this is an infinitesimal quantity, it might cause a metal contamination in a manufacturing line. Therefore, it is effective to perform the pre-coating process on the surface of the mounting table 9.
There is a case of performing a film formation using an organic silane gas, e.g., trimethylsilane (SiH(CH3)3) gas as a processing gas. For example, a formation of a SiCN film (carbon and nitrogen containing silicon film) is performed by adding nitrogen (N2) gas to the trimethylsilane gas. Here, the ceramic plate made of the aluminum nitride is obtained through the steps of adding and mixing powders of titanium oxide (TiO) to and with powders of aluminum nitride as a binder, as shown in FIG. 6B; pressing and molding the mixture and sintering them. Thus, problems as follows can occur. If a pre-coating is performed on the SiCN film by using the organic silane gas, TiO contained in the aluminum nitride reacts with H of the trimetyhlsilane (SiH(CH3)3), generating Ti, as indicated by a reaction formula (1) as bellows.TiO+2H→Ti+H2O  (1)
The amount of Ti generated while forming the pre-coat film is at a level that causes no problem. However, since the number of hydrogen atoms contained in a single molecule of the organic silane gas is great, the amount of hydrogen contained (trapped) in the pre-coated film will be also great. Due to the presence of this hydrogen, the Ti is generated with a lapse of time after the completion of the pre-coating. Ti thus generated is transferred to a rear surface of the substrate when the substrate is mounted on the mounting table. For example, as for a mounting table pre-coated with a SiCN film using trimethylsilane, the quantity of Ti transferred to the rear surface of a substrate immediately after the pre-coating process was about 5×1010 atoms/cm2. However, the quantity reached up to about 5×1013 atoms/cm2 in 2 or 3 days.